Oxford astro-ph Coffee

Calculation to 2nd order of the effects of inhomogeneities on averaged observables.

Authors: Lado Samushia,Beth A. Reid,Martin White,Will J. Percival,Antonio J. Cuesta,Lucas Lombriser,Marc Manera,Robert C. Nichol,Donald P. Schneider,Dmitry Bizyaev,Howard Brewington,Elena Malanushenko,Viktor Malanushenko,Daniel Oravetz,Kaike Pan,Audrey Simmons,Alaina Shelden,Stephanie Snedden,Jeremy L. Tinker,Benjamin A. Weaver,Donald G.

Massive Gravity is this year's must-have theory of modified gravity. The concept is simple - what if the graviton had a (very small) mass? However, building a consistent and viable theory from this idea has proved very difficult. It has now been achieved for the background-level cosmology, and can fit the accelerated expansion (with the usual fine-tuning problems, of course!) This paper takes the first steps towards the perturbation theory that needs to be developed if we are to test Massive Gravity with measurements of structure growth, etc.

More dipoles!

Astrophysics > Cosmology and Extragalactic Astrophysics Title: Dipoles in the Sky (Submitted on 29 May 2012) Abstract: We perform observational tests of statistical isotropy using data from large-scale structure surveys spanning a wide range of wavelengths.

Measurement of the homogeneity scale. See Fig. 6. Fractal dimension D_2=3 for homogeneity.

An explanation of the anomalously low measurement of the local dark matter density that we discussed a few weeks ago?

Combining galaxy distribution / clustering measurements with weak lensing shear measurements is a hot topic at the moment and this is the latest attempt to use both to reconstruct the COSMOS density field.

Astrophysics > Cosmology and Extragalactic Astrophysics Title: Large-Scale Structure with Gravitational Waves I: Galaxy Clustering (Submitted on 7 May 2012) Abstract: Observed angular positions and redshifts of large-scale structure tracers such as galaxies are affected by gravitational waves through volume distortion and magnification effects.

You can fix the acoustic scale using low-z luminosity distance measurements instead of the high-z CMB measurement. This would be a useful consistency test for LCDM, and can be used to constrain N_eff.

The chameleon mechanism suppresses the effects of a modified gravity theory in high-density environments. The authors of this paper suggest that this could lead to effects on stellar structure that would impact distance measures inferred from cepheid variables and red giant branch stars. Whilst it's not clear that the astrophysics involved is sufficiently well understood for such tests to be useful, I think there is some good thinking-outside-the-box here!

The authors numerically showed that angular momentum of isolated galactic halos with mass less than 10^11 solar mass is systematically larger in the f(R) gravity model since they are less affected by shielding via chameleon mechanism.

They claim a ~100x speed increase when using GPUs over a single processor. Execution time on the GPUs is comparable with a 128-processor MPI implementation.

Deason et al have an improved measurement of density and velocities of blue horizontal branch stars in the MW halo: here they fit dynamical models an infer a halo of about 1e12 Msun (as before) and quite a high concentration (cvir~20). Since concentration reflects age, this is consistent with the picture in which the MW has been undisturbed for a long time.

Authors note a new phenomenon: linear portions of quasar lightcurves have gradients that appear vary systematically with quasar redshift (higher z quasars have steeper gradients). Interesting if real: it could sharpen up QSO redshift estimation in cadenced imaging surveys beyond what you can do with the QSO colours.

"We extrapolate a dark matter (DM) density in the solar neighborhood of 0+-1 mM_sun pc^-3, and all the current models of a spherical DM halo are excluded at a confidence level higher than 4sigma."

Celia visited us for several months last summer - this paper is the outcome of her work here. In EBI gravity there is an 'Eddington-dominated' epoch in the universe prior to radiation domination, which can avoid the need for a big bang singularity. However, it turns out that tensor perturbations in this early phase are unstable. It's particularly interesting that the instability only shows up at the perturbative level, whilst the background cosmology remains non-singular.